The technical field of this invention is printers and more particularly the electronics of printers that converts input data in the form of a page description file into control signals for the print engine.
Screening is the process of rendering the illusion of continuous-tone pictures on displays that are only capable of producing digital picture elements. In the process of printing images, large gray levels of the input picture have to be simulated by the printing device to reproduce a perfect duplicate of the original image. However, in the printed image the pixel resolution can be limited to that which is perceivable by the eye. Hence by grouping the adjacent pixels it is possible to simulate a continuous tone in the image.
Screening may take place by a threshold method in one of two categories: bi-level threshold screening; and multi-level threshold screening. In bi-level threshold screening the (x,y) coordinates of the input pixel are used to index into a two dimensional m by n matrix. The individual entries in the matrix are gray level thresholds which are compared against the input pixel gray level. A binary value (0 or 1) is output based on the results of the comparison. Multi-level screening indexes into a three dimensional lookup table. This three dimensional lookup table is organized as a two dimensional preference matrix of size M by N. The preference matrix is a repeatable spatial tile in the image space. Each entry of the preference matrix has a number of the tone curve which has to be used for the position of (x,y). The tone curve is the compensation transfer function of the input pixel-gray value range to within range of the printing process. The tone-curve transfer function is quantized based on a set of thresholds and stored in the form of lookup tables. The lookup tables each contain 2b entries for an unscreened input pixel of size b-bits. All the 2b entries contain the corresponding screened output pixel of size c-bits. This process provides a manner of translating the large dynamic range of the input image into the smaller dynamic range of the printer by mixing colors within the printer dynamic range.
A screening method in a printer for approximating a gray scale tone with a more limited range image producer using a tree search. An input pixel value is packed into equal sections of a first data word. This first data word is compared with an initial second data word holding differing threshold values in the sections. A next second data word is selected based in a based upon a result of said simultaneously comparing within a group with a next reduced range. The first data word is again compared with the next second data word. This process repeats until a comparison with a second data word having adjacent threshold values.
The comparing steps are preferably performed by subtracting the second data word from the first data word in a splittable arithmetic logic unit and storing respective carry outs from each section. The selection of the next second data word uses the stored carry outs from each section. The second data words preferably have threshold values packed in order from highest to lowest. The next second data word can be determined by extracting a left most one of the stored carry outs for use as an index into a table. Alternatively, the stored carry outs can be used directly as an index into a table. An output pixel value is determined for each pixel. This may be computed from the stored carry outs or extracted from a table.